A number of processes have been proposed to re-refine used oils. Any successful used oil re-refining process must satisfactorily address the profound tendency of used oil to foul high temperature heat exchanger surfaces and distillation columns. Most commercially successful processes heretofore have dealt with this fouling issue by eliminating high temperature shell and tube heat exchangers and conventional vacuum columns altogether in processing undistilled used oil at elevated temperatures beyond about 500 F, and instead employ wiped or thin film evaporators in their place, not withstanding the well known performance shortcomings of this sort of distillation apparatus. These shortcomings include their mechanical complexity, inability to take side draws, and inability to distill with more than one theoretical plate in a single step. U.S. Pat. No. 4,941,967 represents an exemplary process.
Other prior art processes have sought to deal with the fouling issue by employing some pretreatment modality which seeks to address factors presumed to be the key source of fouling in used oil prior to high temperature vacuum distillation in a substantially conventional vacuum column or its exposure to high temperature shell and tube heat exchangers. U.S. Pat. Nos. 5,286,380, 4,381,992, and 5,306,419 represent exemplary processes. The commercial experience of these processes indicates that these efforts have not heretofore been fully successful. Accordingly, the fouling issue may be more complex and multi-faceted than previously thought, particularly when a wide range of used oils must be processed. In the end, no prior art process has successfully reduced the fouling tendency of used oils to a level consistent with reliable trouble free subsequent distillation in a packed distillation column.
In addition, most prior art re-refining processes have considerable difficulty in processing used oils, such as used metalworking oils, which are high in chlorine due to the presence of chlorinated paraffins. These difficulties stem from the corrosion generally resulting therefrom, the incompatibility of elevated chlorine levels with downstream processing units such as hydrofinishers, and the unacceptability of elevated chlorine levels in most prospective finished re-refined products, such as distilled fuels or re-refined base oils.